Irreversible optical recording medium

ABSTRACT

The medium comprises a bilayer stack constituted of an inorganic layer ( 30 ) and a semi-reflecting layer ( 32 ). The inorganic layer ( 30 ) can be deformed under the effect of light radiation ( 34 ) passed through the semi-reflecting layer, which lowers the reflection coefficient of the stack. 
     Application to irreversible recording of information data, for example on discs.

TECHNICAL FIELD

The present invention relates to irreversible optical recording media,for the writing and reading of information data respectively. Therecording is irreversible in the meaning that the writing only takesplace once, without any possibility of either effacing or rewriting. Onthe other hand, reading can be repeated.

The invention can be applied, for example, to optical disc recording ofthe CD-R (Compact Disc Recordable), WORM (Write-Once Read Many), DRAW(Direct Read After Write) or DVD-R (Digital Versatile Disc Recordable)type.

But the invention is not limited to the case of discs. It covers anymedium, of any form whatsoever (tape, card etc.).

STATE OF PRIOR ART

In the field of irreversible optical recording, the CD-R, compatiblewith CD (Compact Disc) quickly imposed itself as a world standard. Theevolution of world-wide production is clear evidence of this phenomenon:862 million discs in 1998, 2078 million in 1999. The present price of aCD-R is less than $1 and a reduction of 30% is expected in the two yearsto come. The CD-R is thus no longer used only for personal recording butalso for producing small series of discs, instead of pressing CDs orCD-ROMs. The length of economic life of the product is difficult toestimate. The most optimistic think that the CD-R may not suffer fromcompetition with the CD-RW (Compact Disc Readable Writable) and the DVDfamily. In particular, in the computing field, the CD-R could replacethe diskette definitively, having a capacity 400 times greater.

At present, the great majority of discs use a technique with a base oforganic colouring. The structure of the disc is shown in FIG. 1.

The recording medium shown comprises:

-   -   a transparent substrate 10 (in polycarbonate for example),    -   a layer 12 of colouring sensitive to wavelengths comprised        between 7750 and 7950 Å,    -   a reflecting layer 14, in gold or silver alloy for example,    -   one or two protective layers 16.

The data are written on the disc by focussing an optical beam 18 emittedby a high power laser. The beam reaches the layer 12 of colorant throughthe substrate 10. The light absorbed by the colorant heats the latterand produces an irreversible modification of the optical structure. Thismodification may be limited to the colorant, but it may also reach thesurrounding materials: substrate 10 or reflecting layer 14.

The data are recorded under the form of alternation of non-written zoneswith high reflection and written zones with low reflection. The datacoding is obtained by variation of the length of the marks inscribed.

Data reading is obtained with a light beam reader from a low power laserdiode.

The data are written in a spiral on the disc surface. This follow-up ofthis spiral is made possible, both for reading and for writing, throughthe presence of a pre-engraved groove in the substrate. The data areonly written in the groove. The virgin zone of the spiral is called theland.

This technology has a certain number of problems linked with the ageingof the organic products (by light or temperature for example), with thehigh sensitivity to wavelength of the colorant (compatibility problembetween CD and DVD type formats for example), and with the cost of thestage of depositing the colorant (length of time, maintenance, cost ofthe raw material).

The latter problem, especially, increases the interest of anothersolution enabling reduction of production costs. In particular, incountries where the cost of work-hours is high, it is difficult tofollow market trends. Thus, in 1998, 37% of world production came fromTaiwan compared to 60% in 1999. An interesting solution would be toreplace the organic product by an inorganic product. But then otherproblems arise:

-   -   passage from high reflection, for the virgin disc, to low        reflection for the written points must be preserved to allow        compatibility with existing material (engravers and readers),    -   sufficient sensitivity must be preserved so that the writing        power remains within the range accessible to present engravers        at a given engraving speed,    -   adequate writing quality (strong signal-to-noise ratio, low        jitter, low asymmetry) must be obtained to guarantee        satisfactory writing and reading,    -   the service life must be sufficient despite ageing due to        temperature, the sun, impacts and scratches.

It is difficult to solve all these problems simultaneously. Furthermore,industrial realities impose the use of tested and rapid deposittechnologies. Usually this involves pulverisation and not evaporation,for example, often too slow. In the same way, cost reduction involvesminimising the number of pulverisation stations and therefore the numberof materials and layers, and even the thickness of the latter.

The use of inorganic materials for irreversible recordings has beenunder consideration for a long time. Tellurium and its alloys werestudied even before the appearance of colorants. Since the latterimposed themselves, new materials appear regularly in this domain. Amongthese solutions, few are compatible with existing material for engravingand reading. Nonetheless, many varied solutions have been studied [1]:

Hole formation [2]: This concerns the method studied the most in theeighties. It consists of creating a hole in the active layer with theaid of an intense light pulse. The most used materials in this techniqueare In, Bi, Te and various chalcogenides (excluding the use of organicmaterials). All these materials have in common a low fusion point andhigh absorption properties.

Bubble formation [3]: Traditionally, the active layer comprises ametallic layer (gold or platinum alloy) and a layer of organic polymer.The rise in temperature in the metallic layer is transferred to thepolymer layer, resulting in decomposition of the latter and gasemission. This gas detaches the metallic layer from the substrate.Bursting of the bubble can be avoided by optimising the parameters ofthe active layer. Work on AgO_(x) can also be included in this category.

Segregation [4]: The constituent of the layer decomposes under laserirradiation. The sub-oxides are suitable candidates, for exampleTeO_(1,1). This material decomposes into TeO₂ and Te after irradiation.Since Te is reflecting, the written point is more highly reflecting thanits surroundings.

Crystalline-amorphous phase change: certain materials studied within theframework of ablation are close to phase-change materials. Thus it canbe deduced that the materials usually used for recording by change ofphase can be used in irreversible manner within the framework ofablation. However, to ensure that data writing by phase changerepresents a modification of reflection from a high to a low value, thematerial must change from the crystalline state to the amorphous state.But the material is in an amorphous state after being deposited on thesubstrate. Writing by phase change therefore requires an initialisationstage consisting of crystallising the material over the whole surface ofthe disc. This stage involves a non-negligible cost factor in themanufacture of a disc of this type. Finally, control of the thermaleffects in the disc often needs the presence of three to four layers,certain of them, dielectric, being relatively thick. The use ofirreversible writing based on phase change therefore seemed to bewithout interest from the economic point of view.

Texture change [5]: This technique, in general, concerns active layerswith germanium or silicon base. The rough surface of the layer becomessmooth after laser irradiation. Reflection therefore changes from a lowvalue to a high value. This is not the change direction required.Furthermore, these discs often have a high noise level.

As far as the Applicants know, a disc with reflection greater than 60%with a single layer of inorganic material can only be obtained withthick layers and/or materials close to the noble metals. But writing inthese materials can only be carried out at high powers incompatible withstandards, in particular because of lower absorption, higher thermalconductivity and a higher fusion temperature. For example, it isimpossible to write a so-called “3T-3T” signal (where 3T indicates thelength of the marks written on the disc and the length of the intervalsbetween the marks), with a good signal-to-noise ratio (this ratioremaining lower than 20 dB whatever the power) in a layer of gold of 20nm with a reflection of 60%. The same trial with a layer of 10 nm, witha reflection of around 40%, produces an identical result. Generally, toraise sensitivity, materials more sensitive than noble metals are used,for example tellurium. Unfortunately, a single layer of sensitivematerial generally does not make it possible to obtain sufficientinitial reflection. This is also the case with a material such astellurium used by the Applicants as shown in FIG. 2. This figure shows,as abscissa, the reflection R in % in a writing zone and as ordinate,the minimum writing power P (in mW). The obtained reflection for thematerial under study never exceeds 50%, whatsoever the thicknesses anddeposit conditions.

A traditional method for increasing the reflection consists of adding alayer of gold or silver behind the sensitive layer [6] (relative to theincidence of the light, similar to the case of discs with a base oforganic material). This reflecting layer is generally separated from thesensitive layer by a dielectric layer. For certain writing mechanisms,such as hole formation for example, the presence of this dielectriclayer often results in a loss of sensitivity and a drop in thesignal-to-noise ratio. Therefore, it is necessary to reduce thethickness of the sensitive layer in order to lower the threshold power.Unfortunately, in general, very thin layers have a poor signal-to-noiseratio and a lower reflection.

The aim of the present invention is to overcome all these disadvantages.

DESCRIPTION OF THE INVENTION

The invention therefore recommends using a very simple structure,without organic material, with operation compatible with knowntechniques and materials. Essentially, this structure comprises abilayer stack constituted of a semi-reflecting layer and an inorganiclayer. The writing is obtained through the semi-reflecting layer. Thereading takes place by the same face. Under the effect of irradiation,the inorganic layer suffers deformations of diverse natures: hollows,cracks, bubbles, cavities, craters, bulges, splits, swellings, curling,partial or total ablation, reflux of material etc. There may also bedeformations in the semi-reflecting layer and in the substrate. Thepower levels required for deformations to appear are compatibles withthose operating in present engraving equipment. In any case, they havethe effect of lowering the reflection coefficient of the bilayer stackwhich, outside the zones deformed in this way, can reach 65%. Theinorganic layer at the origin of these deformations is hereinaftercalled the “active layer”. In itself, when it is not deformed, it has acertain reflection coefficient which is insufficient. Thesemi-reflecting layer acts together with the inorganic layer to raisethe value of this reflection.

More precisely, the aim of the present invention is a recording mediumfor writing information data, characterised in that it comprises abilayer stack constituted by a semi-reflecting layer and an inorganicactive layer, the inorganic active layer being suitable for undergoingdeformations under the effect of an optical radiation for writingdirected through the semi-reflecting layer, these deformations loweringthe reflection coefficient of the stack.

A further aim of the invention is a recording medium for readinginformation data, characterised in that it comprises a bilayer stackconstituted by a semi-reflecting layer and an inorganic active layer,the inorganic active layer having deformations in certain zones, thereflection coefficient of the stack being lower inside these zones thanoutside them.

Preferably, the bilayer stack is deposited on a substrate. Thesemi-reflecting layer can be set between the substrate and the inorganicactive layer. Conversely, the organic active layer can be set betweenthe substrate and the semi-reflecting layer. In the former case, thesubstrate must be transparent. This substrate can be engraved in theform of a spiral groove.

More preferably, the semi-reflecting layer is a metal. This metal can beselected from the group comprising aluminium, silver, copper, gold,zinc, titanium and their alloys. Aluminium seems to be a particularlyappropriate metal.

The semi-reflecting layer can be produced in two (or more) differentmaterials. For example, it may comprise a thin layer of gold and a thinlayer of silver.

Preferably, the active layer is made in a material selected from thegroup comprising tellurium, antimony, selenium, indium, bismuth, arsenicand their alloys.

The active layer can be an SbTe or SbSe alloy with a metal selected fromthe group comprising Al, Ag, Cu, Si, As.

Furthermore, the material of the active layer can comprise a certainproportion of nitrogen.

According to an advantageous embodiment, a protective layer is depositedon the stack, on the inorganic active layer side. This protective layercan be made of elastomer-silicon. A dielectric intermediary layer,organic or inorganic, may possibly be inserted between the active layerand the protective layer. This intermediary layer helps writing and/orthe service life of the medium due to its thermal, chemical and/ormechanical properties.

Another aim of the present invention is a writing method for a recordingmedium such as defined hereabove and wherein the optical beam isdirected onto the active layer through the semi-reflecting layer, thepower of the optical beam being able to provoke deformations of theactive layer.

Yet another aim of the present invention is a reading method for arecording medium such as defined above, wherein an optical beam isdirected onto the active layer through the semi-reflecting layer, thepower of the optical beam being able to produce a reflected beam withintensity depending on the deformations of the active layer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1, already described above, shows a diagram of the principle of adisc with organic colouring;

FIG. 2 shows the link between the reflection coefficient of a singleinorganic layer made of Sb₂Te₃ and the luminous power required forwriting;

FIGS. 3A and 3B show diagrammatically the bilayer stack according to theinvention, before and after writing;

FIG. 4 shows a special embodiment of production of a medium according tothe invention in the case of a disc comprising grooves engraved in asubstrate with protective layer;

FIG. 5 assembles the curves showing the development of thesignal-to-noise ratio in function of the writing power for severalthicknesses of the semi-reflecting layer;

FIGS. 6A and 6B show the image of a medium after illumination with alaser of low and high power respectively.

FIG. 7 represents an image of points inscribed on a varnished disc.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3A shows a bilayer stack according to the invention. This stack isconstituted of an inorganic layer 30, hereinafter called “active layer”,and a semi-reflecting layer 32, metallic for example. This stack can bewritten by a light beam 34 directed onto and through the semi-reflectinglayer 32. The energy deposited by the light beam, after crossing thesemi-reflecting layer, deforms the active layer as explained above. Thesemi-reflecting layer 32 may or may not be deformed as well.

FIG. 3B shows the stack after writing and during the reading operation.The reading beam 36, of lower intensity than the writing beam 34, isdirected towards the semi-reflecting layer 32. Due to the deformation ofthe active layer 30 (which in the case shown is linked to the appearanceof a hole 33) and possibly of the semi-reflecting layer 32, thereflection coefficient of the stack is lower in the written zones Zethan in the non-written zones Zne. The reflected beam 38 is thereforeless intense in the written zones Ze than in the non-written zones Znewhich makes it possible, by optical means known to those skilled in theart, to differentiate between the different zones and thus to read therecorded data.

The bilayer stack described above can be deposited on any substratewhatsoever and in particular on a substrate in disc form with enengraved groove. This is shown in FIG. 4 with a certain number ofoptions. In this figure, one can see a transparent substrate 40 engravedwith grooves 42 and a bilayer stack 30–32 according to the invention.

Furthermore, in the embodiment shown, the medium comprises a dielectriclayer 44 and a protective layer 46.

The substrate 40 can be made in plastic (polycarbonate or PMMA, forexample) with grooves of width of from 400 to 800 nm and depth of from20 to 60 nm.

In FIG. 4, the semi-reflecting layer 32 is deposited on the substrate,which means that the latter is assumed to be transparent, because boththe writing and reading take place through this layer. But thearrangement can also be reversed and the active layer can be set on thesubstrate, in which case it is the protective layer 46 which must betransparent. In this case, the substrate can be opaque.

The material of the semi-reflecting layer is chosen for its reflectingproperties (Al, Zn, Au, Ag, Cu or their alloys for example) . It ispreferable for this layer to absorb little light. The semi-reflectinglayer being seen first by the light beam, its thickness must be adjustedas accurately as possible to increase the reflection without raising thewriting threshold excessively. This thickness can, for example, be offrom 4 to 10 nm.

The active layer is at the base of the writing mechanism but alsoparticipates in the reflection of the stack. Its thickness is of from 10to 100 nm and must be adjusted to be able to conserve reasonable writingpower with adequate reflection. The conditions for depositing this layerare adjusted in order to work with optimum thickness with constantreflection. In fact, the holes, bubbles, cavities etc. formed must besufficiently big for the contrast to be good but not too big in order tolimit the reading noise. However, the size of the holes, bubbles,cavities etc. seems to be proportional, in certain cases, to thethickness of the layer. The material of this layer can, for example, be:Te, Sb₂Te₃, In, Bi, Bi₂Te₃, SeTe, Se, As₃Se₃, As₂Te₃.

The measurement results are shown in FIG. 5. This figure shows thevariations of the signal-to-noise ratio (S/B) as ordinate and expressedin dB, in function of the optical power P as abscissa and expressed inmW. The curves correspond to a succession of written zones of width 3Tand non-written zones of the same width, for a linear rotation speed ofthe disc equal to four times a reference speed of 1.2 m/sec, that is 4.8m/sec. The active layer had a thickness of 25 nm. The semi-reflectinglayer was a layer of aluminium of variable thickness, of 6 nm, 7 nm, 8nm and 9 nm respectively for the curves 51, 52, 53, 54. The reflectioncoefficient of the stack in the non-written zones was 46, 48, 50 and 51%respectively (whereas, without a reflecting layer, it would only havebeen 40%)

The bilayer stack according to the invention suffices to write and readinformation data. Nonetheless, as an option, it is possible to add otherlayers to this stack to protect the disc from physico-chemicalmodifications of the materials (oxidation for example), to avoid damageby scratches or impacts and to avoid soiling or various traces whosepresence could hinder the reading or writing and whose cleaning couldrisk damaging the disc. Therefore one or two layers can be added, oreven more, knowing that the greater the number of layers added the lessinteresting the solution becomes economically. In general, the lastlayer deposited ensures mechanical protection. It is often a varnishdeposited using a turntable, solidified under ultraviolet radiation, andwith a thickness of a few microns. The sealed disc technique can also beused.

For various reasons, the intermediary dielectric layer 44 can be addedbetween the active layer 30 and the protective layer 46, as shown inFIG. 5, in particular to encourage deformation of the active layermechanically and thermally and to strengthen its resistance. Thisintermediary layer can also insulate the active layer chemically fromthe protective layer.

Preferably, the active and reflecting layers are deposited bypulverisation. The conditions for depositing the two layers are chosenso as to obtain sufficient reflection while still conserving a writingpower threshold compatible with most engraving equipment available onthe market. The intermediary and protective layers can be produced bypulverisation or by turntable deposit, depending on the nature of thematerials and the thickness of the layers.

A few example of embodiments will now be described. It is evidentlyunderstood that these do not limit the scope of the invention in any waywhatsoever.

EXAMPLE 1

The substrate is in polycarbonate. The reflecting layer is in aluminiumwith a thickness of from 6 to 9 nm. It is deposited by pulverisationwith a target current of 500 mA and an argon pressure of 3.10⁻³ mbar.The active layer is constituted of Sb₂Te₃, of thickness 25 nm(calculated from deposit time). It is deposited by pulverisation with anargon pressure of 10⁻³ mbar and a target current of 100 mA. The focussedbeam reflections measured using a CD-R tester available on the marketare as follows:

Sample number 1 2 3 4 5 Aluminium thickness (nm) 0 6 7 8 9 Reflection(arbitrary unit) 1 1.15 1.2 1.25 1.28

Variations in the signal-to-noise ratio in function of the writing powerobtained at speed 4.8 m/sec for samples comprising an aluminium layerare shown in FIG. 6. For a speed of 1.2 m/sec the signal-to-noise ratiois higher than 45 dB for a power greater than 5.5 mW for samples 2 and3, and greater than 7 mW for sample 4. The signal-to-noise ratio ishigher than 43 dB from 8 mW for sample 5. At double speed (2.4 m/sec)the signal-to-noise ratio is greater than 47 dB from 7.5 mW for samples2 and 3, and from 10 mW for sample 4.

EXAMPLE 2

The substrate is in polycarbonate. The reflecting layer is in aluminiumwith a thickness of from 6 to 9 nm. It is deposited by pulverisationwith a target current of 500 mA and an argon pressure of 3.10⁻³ mbar.The active layer is constituted of Sb₂Te₃, of thickness 30 nm(calculated from deposit time). It is deposited by pulverisation with anargon pressure of 10⁻³ mbar and a target current of 250 mA. The focussedbeam reflections measured using a CD-R tester available on the marketare as follows:

Sample number 1 2 3 4 5 Aluminium thickness (nm) 0 6 7 8 9 Reflection(arbitrary unit) 1 1.12 1.19 1.23 1.30

Sample 2 has a signal-to-noise ratio greater than 42 dB starting from 4mW at speed 1.2 m/sec, 46 dB starting from 6 mW at speed 2.4 m/sec and48 dB starting from 10 mW at speed 4.8 m/sec. Sample 3 has asignal-to-noise ratio greater than 45 dB starting from 6.5 mW at speed1.2 m/sec, 8.5 mW at double speed and 12 mW at quadruple speed. Allthese tests were carried out with a signal composed of alternation ofzones of the same length.

EXAMPLE 3

The substrate is in polycarbonate. The layer is in aluminium of 7 nmthickness, the active layer is of thickness 40 nm and is constituted ofSb₂Te₃. A protective layer of 40 nm in silica is provided. Thereflection of the disc in non-written zone is 60%. This stack wasirradiated by a light beam of wavelength comprised between 7700 Å and7900 Å and the results of the irradiation were observed by AFM (AtomicForce Microscope). The disc surface (silicon-layer side), for low powerand for high power, is shown in FIGS. 6A and 6B.

EXAMPLE 4

The substrate is in polycarbonate. The active layer of 20 nm is composedof Sb₂Te₃. It is produced by pulverisation. The semi-transparent layeris in aluminium-chrome alloy of 7 nm thickness. A protective varnish wasdeposited by turntable and solidified with a UV lamp. FIG. 7 shows thepoints inscribed on the disc.

EXAMPLE 5

The substrate is in polycarbonate. The active layer of 28 nm is composedof Sb₂Te₃. It is produced by pulverisation. The semi-transparent layeris in aluminium of 6 nm thickness. The signal-to-noise ratio wasmeasured before application of a protective layer. The threshold powersare compatible with the CD-R standard: 4 mW at speed 2.4 m/sec, 5 mW at4.8 m/sec, 8 mW at 9.6 m/sec.

With an elastomer layer, the threshold powers remain compatible with theCD-R standard: 4 mW at speed 2.4 m/sec, 5 mW at 4.8 m/sec, and 9 mW at9.6 m/sec.

At the speed 145.4 m/sec the threshold power is 13 mW for asignal-to-noise ratio of 47 dB, with the elastomer layer.

Concerning the wavelength of the light used to write and read the data,certain information can be provided. For the CD family (CD-R) one worksat around 8000 Å (close to the infra-red), the ranges being determinedin the standards. They differ for reading and writing. For the DVD(DVD-R for example) one works in the red around 6300 or 6500 Å. Otherwavelengths, in the green or in the blue can also be used.

REFERENCES

1. “Principles of optical disc systems”, G. Bouwhuis et al., publishedby Adam Hilger Ltd, chapter 6 “Materials for on-line optical recording”p. 210–227.

2. “Ablative hole formation process in thin tellurium-alloy films”, M.Chen et al., appl. Phys. Lett. 41(9), 894–896 (1982).

3. “Thin films for optical data storage”, W.-Y. Lee, J. Vac. Sci.Technol. A3(3) 640–646 (1985).

4 “Laser recording in tellurium suboxide thin films”, Y-S Tyan et al.,J. Appl. Phys. 59(3) p. 716 (1986).

5. Textured germanium optical storage medium”, H. G. Craighead, Appl.Phys. Lett. 40(8) p. 662 (1982).

6. EP 0605891 and EP 0747895.

1. Recording medium for writing information data, comprising: a bilayerstack comprising a semi-reflecting layer, and an inorganic active layer,the inorganic active layer being suitable for undergoing deformationsunder the effect of an optical radiation for writing directed throughthe semi-reflecting layer, these deformations lowering the reflectioncoefficient of the stack.
 2. Recording medium according to claim 1,wherein the bilayer stack is deposited on a transparent substrate, thesemi-reflecting layer being set between the substrate and the inorganicactive layer.
 3. Recording medium according to claim 1, wherein thebilayer stack is deposited on a substrate, the inorganic active layerbeing set between the substrate and the semi-reflecting layer. 4.Recording medium according to claim 1, wherein the semi-reflecting layeris in metal.
 5. Recording medium according to claim 4, wherein the metalof the semi-reflecting layer is selected from the group comprising Al,Ag, Cu, Au, Zn, Ti and their alloys.
 6. Recording medium according toclaim 4, wherein the semi-reflecting layer comprises two metalliclayers.
 7. Recording medium according to claim 1, wherein the inorganicactive layer is in a material selected from the group comprising Te, Sbor Se, and their alloys.
 8. Recording medium according to claim 1,wherein the inorganic active layer is in an SbTe alloy with an elementselected from the group comprising Al, Ag, Cu, Si, As.
 9. Recordingmedium according to claim 1, wherein the inorganic active layer is in anSbSe alloy with an element selected from the group comprising Al, Ag,Cu, Si, As.
 10. Recording medium according to claim 1, wherein theinorganic active layer is in a SeTe alloy with an element selected fromthe group comprising Al, Ag, Cu, Si, As.
 11. Recording medium accordingto claim 1, wherein the inorganic material of the active layer comprisesa proportion of nitrogen.
 12. Recording medium according to claim 1,wherein a protective layer is deposited on the stack.
 13. Recordingmedium according to claim 12, furthermore comprising a dielectricintermediary layer between the stack and the protective layer. 14.Recording medium according to claim 12, wherein the protective layer isin elastomer-silicon.
 15. Recording medium according to claim 1, whereinthe semi-reflecting layer has a thickness of from 4 to 10 nm. 16.Recording medium according to claim 1, wherein the inorganic activelayer has a thickness of from 10 to 100 nm.
 17. Method of writing for arecording medium according to claim 1, wherein an optical beam isdirected onto the active layer through the semi-reflecting layer, thepower of the optical beam being able to provoke deformations of theactive layer.
 18. Recording medium according to claim 1, wherein saiddeformations are structural deformations of the inorganic layer. 19.Recording medium according to claim 18, wherein said deformations areselected from the group consisting of hollows, cracks, bubbles,cavities, craters, bulges, splits, swellings, curling, partial ablation,total ablation, reflux of material and combinations thereof. 20.Recording medium for reading information data, comprising: a bilayerstack comprising a semi-reflecting layer, and an inorganic active layer,the inorganic active layer having deformations in certain zones, thereflection coefficient of the stack being lower inside these zones thanoutside them.
 21. Method of reading for a recording medium according toclaim 20, wherein the optical beam is directed onto the active layerthrough the semi-reflecting layer, the power of the optical beam beingable to produce a reflected beam with intensity depending on thedeformations of the active layer.
 22. Recording medium according toclaim 20, wherein the bilayer stack is deposited on a transparentsubstrate, the semi-reflecting layer being set between the substrate andthe inorganic active layer.
 23. Recording medium according to claim 20,wherein the bilayer stack is deposited on a substrate, the inorganicactive layer being set between the substrate and the semi-reflectinglayer.
 24. Recording medium according to claim 20, wherein thesemi-reflecting layer is in metal.
 25. Recording medium according toclaim 24, wherein the metal of the semi-reflecting layer is selectedfrom the group comprising Al, Ag, Cu, Au, Zn, Ti and their alloys. 26.Recording medium according to claim 24, wherein the semi-reflectinglayer comprises two metallic layers.
 27. Recording medium according toclaim 20, wherein the inorganic active layer is in a material selectedfrom the group comprising Te, Sb or Se, and their alloys.
 28. Recordingmedium according to claim 20, wherein the inorganic active layer is inan SbTe alloy with an element selected from the group comprising Al, Ag,Cu, Si, As.
 29. Recording medium according to claim 20, wherein theinorganic active layer is in an SbSe alloy with an element selected fromthe group comprising Al, Ag, Cu, Si, As.
 30. Recording medium accordingto claim 20, wherein the inorganic active layer is in an SeTe alloy withan element selected from the group comprising Al, Ag, Cu, Si, As. 31.Recording medium according to claim 20, wherein the inorganic materialof the active layer comprises a proportion of nitrogen.
 32. Recordingmedium according to claim 20, wherein a protective layer is deposited onthe stack.
 33. Recording medium according to claim 32, furthermorecomprising a dielectric intermediary layer between the stack and theprotective layer.
 34. Recording medium according to claim 20, whereinsaid deformations are structural deformations of the inorganic layer.35. Recording medium according to claim 34, wherein said deformationsare selected from the group consisting of hollows, cracks, bubbles,cavities, craters, bulges, splits, swellings, curling, partial ablation,total ablation, reflux of material and combinations thereof.